The present invention relates to a nonwoven fabric of chemically bonded non-cellulose fibers having improved water tensile properties. More particularly, the present invention relates to a nonwoven fabric of non-cellulose fibers including an essentially formaldehyde free latex binder capable of providing improved water tensile properties.
A nonwoven fabric is a web or continuous sheet of fibers laid down mechanically. The fibers may be deposited in a random manner or oriented in one direction. Most widely used fibers include cellulosics, polyamides, polyesters, polypropylene and polyethylene. The spun fibers, which may be drawn, are laid down directly onto a porous belt by carding, airlaying or wet-laying, often with the aid of an electrostatic charge. The sheet is then bonded together with a binder subsequently treated in an oven or a calendar to complete the bonding process.
A number of methods have been developed for applying a binder to randomly-dispersed fibers. Typically, a water based emulsion binder system is used in which a thermoplastic or thermoset synthetic polymer latex is prepared and a loose web of fibers to be treated is immersed therein, saturated or sprayed using special equipment in view of the structural weakness of the web; the thus treated web is dried and cured to effect proper bonding. Alternatively, an aqueous or solvent solution binder system of a thermoplastic or thermoset resin may be used to impregnate the fibrous web.
Still other methods include the application of thermoplastic or thermoset resin powders to the fibers, before or after making a web of the same, and passing the web through hot rolls or a hot press to bind the fibers together. Alternatively, thermoplastic fibers having a-softening point below that of the base fibers may be interdispersed in a web of the latter and sufficient heat and pressure applied, such as by the use of heated rolls, to soften the thermoplastic fibers and bind the fiber network together.
Commonly used lattices for non-woven fabrics are those prepared from polymers of butadiene-styrene, butadiene-acrylonitrile, vinyl acetate, acrylic monomers such as methyl acrylate, ethyl acrylate, methyl methacrylate and the like. While the emulsion binder system is the most popular method of forming non-woven fabrics, the homopolymers, copolymers and terpolymers heretofore used therein have suffered from one or more disadvantages. To be useful as a textile material, the synthetic polymer must possess several physical properties. The desired properties include adequate tensile strength over a fairly wide temperature range, a high modulus or stiffness under certain conditions, and good textile qualities such as tenacity, handle and drape.
It will be appreciated that it has been an accepted practice to use self crosslinking or melamine formaldehyde resin posted lattices to give improved water tensiles to a nonwoven non-cellulose product. These systems, however, contain and liberate formaldehyde during the dry/cure cycle. In addition, essentially all commercial self crosslinking and melamine posted lattices require a temperature of at least 280xc2x0 F. and preferably 300xc2x0 F. for proper crosslinking. However, it will be appreciated that because the melting point of many non-cellulose fibers is below the temperature required for proper crosslinking, e.g., polypropylene is around 250xc2x0 F., conventional lattices cannot be used. Accordingly, polypropylene fiber in the nonwoven industry has never enjoyed large success. The problem has been in the specific development of a suitable latex binder to give acceptable tensile properties.
It is an object of the present invention to provide a nonwoven fabric of chemically bonded non-cellulose fibers. Another object of the present invention is to provide a nonwoven fabric including a random arrangement of non-cellulose fibers and an essentially formaldehyde free latex binder capable of developing maximum tensile properties at temperatures less than the melt bonding temperature of the non-cellulose fibers. Yet another object of the present invention is to provide a nonwoven fabric including a random arrangement of non-callulose fibers and an essentially formaldehyde free latex binder capable of providing improved water tensile properties. It is another object of the present invention to provide a nonwoven fabric of chemically bonded non-cellulose fibers that is simple and economical to manufacture.
Briefly, according to this invention there is provided a nonwoven fabric including a random arrangement of non-cellulose fibers and an essentially formaldehyde free latex binder. The latex binder includes a polymer latex prepared by emulsion polymerization of a monomeric mixture in the presence of a polymeric surfactant. The monomeric mixture consists of a conjugated diene monomer, a vinyl substituted aromatic monomer and a vinyl cyanide monomer. The conjugated diene monomer may be selected from piperylene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-butadiene. The vinyl substituted aromatic monomer may be selected from xcex1-methyl styrene, p-tertiary butyl styrene, m-vinyl toluene, p-vinyl toluene, 3-ethyl styrene and styrene. The vinyl cyanide monomer may be selected from acrylonitrile, methacrylonitrile, ethacrylonitrile and phenylacrylonitrile.
The polymeric surfactant is about 15-35 wt % on a dry latex basis. The polymeric surfactant contains about 25-27 wt % styrene/acrylic acid/xcex1-methyl styrene copolymer in water neutralized with about 6-7 wt % ammonium hydroxide.
The essentially formaldehyde free latex binder contains at least about 6.7 wt % vinyl cyanide monomer to bond said non-cellulose fibers and form a nonwoven fabric capable of retaining at least about 78% wet tensile strength measured in the cross direction. Alternatively, the nonwoven fabric of chemically bonded non-cellulose fibers has at least a 10% improvement in wet tensile strength over a comparable, the same type, nonwoven fabric having substantially the same monomeric formulation of essentially formaldehyde free latex binder but free of vinyl cyanide monomer.
Suitable non-cellulose fibers include glass fibers or fibers made from high polymers. The high polymers include polyolefins, polyesters, and acrylics, polyamides and the like. The polyolefin fibers include polypropylene, polyethylene, polybutene and their copolymers. The polyester fibers include any long chain synthetic polymer composed of at least 85% by weight of an ester of a dihydric alcohol and terephthalic acid such as polyethylene terephthalate, and, in addition liquid crystal polyesters, thermotropic polyesters and the like. The acrylic fibers include any fiber forming substance containing a long chain synthetic polymer composed of at least 85% by weight acrylonitrile units xe2x80x94CH2CH(CN)xe2x80x94. It will be appreciated that other types of non-cellulose fibers may also be employed in accordance with the teachings of the present invention. For example, high modulus fibers more commonly known as graphite fibers made from rayon, polyacrylonitrile or petroleum pitch may also be used.
The nonwoven fabric of non-cellulose fibers is formed by providing a random arrangement of non-cellulose fibers. Next, an essentially formaldehyde free latex binder is applied to the fibers. Then the latex binder is heat treated to chemically bond the non-cellulose fibers to form a dimensionally stable nonwoven fabric.
The present invention relates to a nonwoven fabric of chemically bonded non-cellulose fibers. The fabric may be used for soft and drapable fabrics such as diaper cover stock, feminine hygiene cover stock, medical gowns, masks, caps and drapes, and for stiff and resilient fabrics such as apparel interliners, furniture skirting, quilts, water bed baffles and clothing insulation and padding.
The fabric of the present invention is made by forming a mat of randomly arranged non-cellulose fibers which are chemically bonded by an essentially formaldehyde free latex binder. The essentially formaldehyde free latex binder is capable of chemically bonding the non-cellulose fibers and forming a dimensionally stable nonwoven fabric. As well known in the art, the latex binder may be applied to the layer of randomly arranged non-cellulose fibers in a spaced, intermittent pattern of binder sites, or uniformly applied throughout the layer of non-cellulose fibers.
As used herein the term xe2x80x9cessentially formaldehyde freexe2x80x9d refers to a latex binder which does not liberate more than 0.7 (PPM) parts formaldehyde per million parts of latex binder during the conventional dry/cure cycle of the latex binder as determined by the Nash/HPLC method (high performance liquid chromatography) as well known in the art and the term xe2x80x9cchemically bondedxe2x80x9d as used herein refers to a bond that is not formed as a result of a heat treatment, for example, as by melt bonding as evidenced by a physical change in the fibers.
The non-cellulose fibers of the fabric may be glass fibers or fibers made from high polymers. The glass fibers are of a type well known in the art and manufactured of molten glass extruded through small orifices and then spun at high speeds. Suitable high polymers include polyolefins, polyesters, and acrylics, polyamides and the like. The polyolefin fibers include polypropylene, polyethylene, polybutene and their copolymers. The polyester fibers include any long chain synthetic polymer composed of at least 85% by weight of an ester of a dihydric alcohol and terephthalic acid such as polyethylene terephthalate, and, in addition liquid crystal polyesters, thermotropic polyesters and the like. The acrylic fibers include any fiber forming substance containing a long chain synthetic polymer composed of at least 85% by weight acrylonitrile units xe2x80x94CH2CH(CN)xe2x80x94. It will be appreciated that other types of non-cellulose fibers may also be employed in accordance with the teachings of the present invention. For example, high modulus fibers more commonly known as graphite fibers made from rayon, polyacrylonitrile or petroleum pitch may also be used.
The non-cellulose fibers may be of most any suitable size and randomly arranged to most any suitable thickness depending upon the desired end use of the nonwoven fabric. The non-cellulose fibers are typically of a length of about 0.25 to 2 inches and typically about 1.2-6 denier. The non-cellulose fibers may be laid in an overlapping, intersecting random arrangement to a thickness of about 0.25 inches or less to form a mat of non-cellulose fibers. The non-cellulose fibers may be arranged by most any convenient known manner such as by wet laying, air-laying or carding.
After the non-cellulose fibers are randomly arranged as desired, a latex binder is applied to the fibers. The latex binder is employed in an effective amount which will result in the resulting fabric having sufficient strength and cohesiveness for the intended end use application. It will be appreciated that the exact amount of the latex binder employed depends, in part, upon factors such as the type of fiber, weight of fibrous layer, nature of latex binder and the like. For example, end uses which require a stronger fabric may utilize more binder. A typical content of latex binder applied on a non-cellulose fiber mat is about 15-40 wt %. It is preferred that the minimum amount of latex binder be applied to obtain the minimum desired required physical properties of the nonwoven fabric such as tensile, hand and the like as well known in the art.
The latex binder utilized in accordance with the present invention may be prepared by well-known conventional emulsion polymerization techniques using one or more ethylenically unsaturated monomers and a polymeric surfactant as herein disclosed and additional conventional additives such as free-radical initiators, optional chain transfer agents, chelating agents and the like can be utilized as set forth in U.S. Pat. No. 5,166,259 to Schmeing and White.
Suitable ethylenically unsaturated monomers in the emulsion polymerization reaction include conjugated diene monomers, vinyl substituted aromatic monomers and vinyl cyanide monomers.
The conjugated diene monomers generally contain 4 to 10 carbon atoms, and preferably 4 to 6 carbon atoms. Examples of specific diene monomers include piperylene, isoprene, 2,3-dimethyl-1,3-butadiene, and the like, and preferably 1,3-butadiene. The amount of conjugated diene monomers utilized is from about 50-70 wt %, preferably from about 55-65 wt %, and most preferably about 60 wt %. The vinyl substituted aromatic monomers generally contain 8 to 12 total carbon atoms. Examples of specific vinyl substituted aromatic monomers include xcex1-methyl styrene, p-tertiary butyl styrene, m-vinyl toluene, p-vinyl toluene, 3-ethyl styrene, and the like, and preferably styrene. The amount of vinyl substituted aromatic monomers utilized is from about 16-50 wt %, preferably from about 27-50 wt %, and most preferably about 27 wt %. It will be appreciated that when the amount of vinyl substituted aromatic monomers in the present invention is greater than about 50 wt %, the latex becomes brittle, is unacceptable as a binder and has unacceptable dry and wet tensile properties for non-cellulose nonwoven fabrics. Moreover, the more conjugated diene monomers and less vinyl substituted aromatic monomers added, generally softer hand feel properties and lower tensile properties are obtained in the non-cellulose nonwoven fabrics. Similarly, the less conjugated diene monomers and more vinyl substituted aromatic monomers added, generally stiffer hand feel properties and higher tensile properties are obtained in the non-cellulose nonwoven fabrics up to an amount that the latex binder does not form a continuous film such that the tensile properties decrease and the non-cellulose nowoven fabrics are too stiff.
The vinyl cyanide monomers may be methacrylonitrile, ethacrylonitrile, phenylacrylonitrile and the like, and preferably acrylonitrile. The amount of vinyl cyanide monomers utilized is at least about 6.7 wt %, preferably from about 6.7-15 wt %, and most preferably about 6.7-10 wt %.
The polymeric surfactant is an acrylic resin neutralized in solution. In a preferred embodiment the polymeric surfactant is a resin containing about 25-27 wt % styrene/acrylic acid/xcex1-methyl styrene copolymer in water neutralized with a base such as a ammonium hydroxide, potassium hydroxide, calcium hydroxide and the like and having an acid value of about 100-300 and a weight average molecular weight greater than about 7,000. Most preferably, the polymeric surfactant is neutralized with about 6-7 wt % ammonium hydroxide and has an acid value of about 205 and a weight average molecular weight of about 8,500 and an average weight ratio of monomers in parts by weight of about 37:32:31 of xcex1-methyl styrene, styrene and acrylic acid.
The resin is prepared in accordance with the process described in U.S. Pat. No. 4,529,787 to Schmidt et al., incorporated herein by reference, using a minor amount of diethylene glycol monoethyl ether as a solvent. Additional resins useful in accordance with the present invention may be made in accordance with the teachings of U.S. Pat. No. 4,414,370 to Hamielec et al. and U.S. Pat. No. 4,546,160 to Brandt et al., incorporated herein by reference.
The amount of polymeric surfactant added to the reactor is typically about 15-35 wt %, preferably about 26 wt % on a dry latex basis, and most preferably, it is believed, about 30 wt %.
The free-radical initiators utilized to polymerize the various above latex binder forming monomers include sodium persulfate, ammonium persulfate, potassium persulfate and the like. Other free radical initiators can be used which decompose or become active at the temperature utilized during polymerization such as various peroxides, e.g., cumene hydroperoxide, dibenzoyl peroxide, diacetyl peroxide,
The optional chain transfer agent can generally be any suitable chain transfer agent well known in the art. Optional chain transfer agents include mercaptans such as the alkyl and/or aryl mercaptans having from 8 to about 18 carbon atoms and preferably from about 12 to about 14 carbon atoms. The tertiary alkyl mercaptans having from 12 to 14 carbon atoms are highly preferred. Examples of suitable mercaptans include n-octyl mercaptan, n-dodecyl mercaptan, t-octyl mercaptan, t-dodecyl mercaptan, tridecyl mercaptan, tetradecyl mercaptan, hexadecyl mercaptan, and the like as well as mixtures thereof. The amount of the chain transfer agent is generally from about 0.2 to about 2.5 parts per hundred parts monomer, preferably 0.4 to about 0.9 parts per hundred parts monomer, more preferably about 0.7 parts per hundred parts monomer. In a preferred embodiment, the chain transfer agent is a dodecyl mercaptan chain transfer agent such as Sulfole 120 commercially available from Phillips 66 Co.
Chelating agents may also be used during polymerization to tie up various metal impurities as well as to achieve a uniform polymerization. The amount of such chelating agents is generally small, such as from about 0.02 to about 0.08, and preferably about 0.05 parts chelating agent per hundred parts total monomer. Examples of suitable chelating agents include ethylene diamine tetraacetic acid, nitrilotriacetic acid, citric acid and their ammonium, potassium and sodium salts. Preferred chelating agents include those chelating agents commercially available under the name Hamp-ene from Hampshire Chemical.
In a preferred embodiment, the polymerization of the ethylenically unsaturated monomers and polymeric surfactant occurs sequentially. The following examples are illustrative of the sequential addition of the ethylenically unsaturated monomers and polymeric surfactant to form the latex binder.